A wind power plant in which a wind turbine generator is installed with a storage battery unit has been well known.
FIG. 5 shows a configuration of the wind power plant. The wind power plant includes a plurality of wind turbine generators 51 at a wind farm, a storage battery unit 55 arranged in parallel with the wind turbine generators, and a grid 61 connected to the wind turbine generators 51 and the storage battery unit 55 via a grid interconnection part 60. Herein, the grid 61 refers to a commercial grid for supplying electric power to houses and so on. In general, the wind turbine generator 51 is configured such that a rotation of a wind turbine blade is inputted to an asynchronous generator 52 via a gear box to generate electric power, and the electric power generated in the asynchronous generator 52 is transformed in an electric transformer 53 and thus inputted to the grid 61 via the grid interconnection part 60.
However, in the wind turbine generator 51, the wind power energy changes in response to change in wind speed, thereby possibly affecting a supply-demand balance and frequency of the grid 61. It is sought to maintain a voltage and a frequency of the electric power supplied to the grid within an allowable range of the total active power and the total reactive power. In the conventional system, the storage battery unit 55 compensates for the shortfall of the electric power beyond the capacity of the wind turbine generator in response to active power and reactive power requested on a grid side. In the storage battery unit 55, a DC-AC converter 57 converts a direct current from a storage battery unit 56 to an alternative current output and then, the converter 58 transforms the converted alternative current via the grid interconnection part and supplies to the grid 61 via the grid interconnection part 60. In this manner, the storage battery unit installed with the wind turbine generator 51 mainly functions to smooth out the output fluctuation of the wind turbine generator 51.
Meanwhile, grid accidents such as lightening strike can generate disturbance in voltage and frequency on the grid side. Conventionally, in the event of such accidents, the wind power plant was disconnected. However, in recent years, it is sought to continue the operation of the wind power plant and restart a steady operation thereof immediately after recovery of the grid without disconnecting the wind power plant from the grid. In addition to this, a new system is studied, in which outputs of active power and reactive power of the wind power plant are simultaneously controlled for the purpose of stabilizing the grid.
For instance, PATENT DOCUMENT 1 (JP 3352662 B) discloses a device to stabilize a grid in the event of grid disturbance. The device includes a first unit for outputting electric power to a grid such as a wind power generator and solar power generator, and a secondary battery system including a battery which is provided with a consideration of over-discharging and charging above a rated value. Further, differences between a setting value of the active power, a setting value of the reactive power, a reference frequency and a reference voltage, and current values thereof are detected. In accordance to the detected references, the active power amount and the reactive power amount outputted from the storage battery unit are controlled based on overloading output and continuation time characteristics.